Image offset only applies to single frame and not to metastrip
You will need to activate a Sequencer addon, that comes included with the official Blender download. 
Extra Sequencer Actions https://wiki.blender.org/index.php/Extensions:2.6/Py/Scripts/Sequencer/Extra_Sequencer_Actions

Change one image to the Offset value you want, then select all the other images and include the changed image last, eg. shift right-click to make this last one active.

Go down to the Strip Menu in the VSE timeline and select Copy Properties > Input - Image Offset



Now all of the other images will inherit the setting from the active strip.